The filter box for overflow filters is located outside the tank. An impellor-driven pump draws water into the filter box via an intake tube and returns it to the aquarium in a larger stream, commonly aimed over the water surface. Most overflow filters contain removable cartridges and space in the filter box for extra filter material. Replaceable cartridges are mostly meant for mechanical filtration, however some may include carbon.
With the addition of proper media, an overflow filter with enough room in the filter box may be converted into an effective biological filter. A suitable biological filter medium will have a broad surface area (without impeding water flow) and should not need replacement.
Extra filter material may be added to an overflow filter as a secondary filter to give further mechanical or chemical filtering. Sponge may be used to offer additional mechanical filtration as well as some biological filtration. Where chemical filtration is needed, carbon, zeolite, or resins may be added.
Overflow filters are typically appropriate for medium to large tanks. Smaller variants often lack the room for additional filtering material and work similarly to an internal power filter. In many circumstances, overflow filters will be preferable over internal power filters since they are often less expensive for the same water turnover. They are also attractive since they sit outside the tank, which is more visually beautiful and frees up more space in the tank for fish and decorations.
The relatively expensive cost of replacement filter cartridges, which must be replaced every few months, is a downside of certain versions. Some variants enable the use of sponge or poly pad instead of replacing cartridges, which lowers recurring expenditures.
Making the best use of this filter system: Choose a model that is appropriate for the size of your tank and the quantity of fish (Remember that good filtration requires turning your tank volume over three times an hour, more filtration may be needed if you have more or larger fish). Consider the space available for additional filter media, as well as if sponge or poly pad may be replaced for replacement filter cartridges if cost is a concern. Extra filter media may be used to meet the demands of your aquarium.
Regularly clean the filter cartridges (or equivalents). If you depend on them for biological filtration, gently rinse them in aquarium water. If you use additional biological filter media, the cartridges may be cleaned with cold tap water, but do not scrape them or they will wear out faster. As required, replace the cartridges (or replacement) and any chemical filter medium. If organic debris accumulates in your biological filter medium, gently rinse it in aquarium water rather than discarding it. If necessary, replacement cartridges may be seeded with nitrifying bacteria by storing them in the aquarium or filter box for a few weeks before use. If your overflow filter has two or more cartridges, you may change them individually rather than all at once.
Clean the area surrounding the impellor every 2-3 months to guarantee smooth operation, and replace the impellor if required.